The move is retroactive to May 17, making Duffy eligible for reinstatement on June 1.
In a corresponding roster move, the team recalled pitcher Brandon Finnegan from Triple-A Omaha.
Duffy, who is 0–3 with a 13.03 ERA in three starts this month, missed Saturday’s start against the St. Louis Cardinals because of shoulder stiffness and was expected to start Tuesday against the New York Yankees.
Jason Vargas will now start Tuesday's game against the Yankees.
Duffy was limited at the end of last season because of a shoulder issue, making only three starts in September and three postseason appearances, all in relief.
Duffy is 2–3 with a 5.87 ERA, 1.70 WHIP, and 30 strikeouts in eight starts this season for the Royals (28–15), who have the best record in baseball and the majors' best bullpen ERA and third-best overall ERA.
- Scooby Axson